Apple and Foxconn both claim to be unaware of any irregularities, and Apple said they're investigating.
I rather think the OP meant something more like their practices surrounding their App Store than this not-quite-substantiated and hyper-sensationalized claim.
They haven't changed because moving away from china would cost them billions and they as most people would rather keep themselves warm with a pile of money than worry about the ethical implications.
But now you've shifted from the part I found dubious--that companies are knowingly engaging in slave labor--to a more clearly established pattern. One can be both horrified by China's treatment of Uigher people and skeptical that Foxconn, Apple, or any one of the 82 companies to which you've referred are complicit.
None of this "excuses" Apple; it's just that there's something faintly disingenuous about focusing on them as if they were a unique source of this problem. We could just as easily say "your Nintendo Switch is manufactured by literal slaves in concentration camps," but we don't, because... it's not as satisfying to be outraged at Nintendo, I guess?
At any rate: I strongly suspect the OP was referring to recent controversy surrounding the way Apple runs the App Store.
https://www.businessinsider.com/apple-forced-uighur-labor-ip...